Network Working Group                                        D. Balenson
Request for Comments: 1423                                           TIS
Obsoletes: 1115                               IAB IRTF PSRG, IETF PEM WG
                                                           February 1993

Privacy Enhancement for Internet Electronic Mail:
Part III: Algorithms, Modes, and Identifiers

Status of This Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This document provides definitions, formats, references, and
   citations for cryptographic algorithms, usage modes, and associated
   identifiers and parameters used in support of Privacy Enhanced Mail
   (PEM) in the Internet community.  It is intended to become one member
   of the set of related PEM RFCs.  This document is organized into four
   primary sections, dealing with message encryption algorithms, message
   integrity check algorithms, symmetric key management algorithms, and
   asymmetric key management algorithms (including both asymmetric
   encryption and asymmetric signature algorithms).

   Some parts of this material are cited by other documents and it is
   anticipated that some of the material herein may be changed, added,
   or replaced without affecting the citing documents.  Therefore,
   algorithm-specific material has been placed into this separate
   document.

   Use of other algorithms and/or modes will require case-by-case study
   to determine applicability and constraints.  The use of additional
   algorithms may be documented first in Prototype or Experimental RFCs.
   As experience is gained, these protocols may be considered for
   incorporation into the standard.  Additional algorithms and modes
   approved for use in PEM in this context will be specified in
   successors to this document.

Acknowledgments

   This specification was initially developed by the Internet Research
   Task Force's Privacy and Security Research Group (IRTF PSRG) and
   subsequently refined based on discussion in the Internet Engineering

   Task Force's Privacy Enhanced Mail Working Group (IETF PEM WG).  John
   Linn contributed significantly to the predecessor of this document
   (RFC 1115).  I would like to thank the members of the PSRG and PEM
   WG, as well as all participants in discussions on the "pem-
   dev@tis.com" mailing list, for their contributions to this document.

      1.  Message Encryption Algorithms ....................... 2
      1.1  DES in CBC Mode (DES-CBC) .......................... 2
      2.  Message Integrity Check Algorithms .................. 4
      2.1  RSA-MD2 Message Digest Algorithm ................... 4
      2.2  RSA-MD5 Message Digest Algorithm ................... 5
      3.  Symmetric Key Management Algorithms ................. 6
      3.1  DES in ECB mode (DES-ECB) .......................... 6
      3.2  DES in EDE mode (DES-EDE) .......................... 7
      4.  Asymmetric Key Management Algorithms ................ 7
      4.1  Asymmetric Keys .................................... 7
      4.1.1  RSA Keys ......................................... 7
      4.2  Asymmetric Encryption Algorithms ..................  9
      4.2.1  RSAEncryption ...................................  9
      4.3  Asymmetric Signature Algorithms ................... 10
      4.3.1  md2WithRSAEncryption ............................ 11
      5.  Descriptive Grammar ................................ 11
      References ............................................. 12
      Patent Statement ....................................... 13
      Security Considerations ................................ 14
      Author's Address ....................................... 14

1. Message Encryption Algorithms

   This section identifies the alternative message encryption algorithms
   and modes that shall be used to encrypt message text and, when
   asymmetric key management is employed in an ENCRYPTED PEM message, for
   encryption of message signatures.  Character string identifiers are
   assigned and any parameters required by the message encryption
   algorithm are defined for incorporation in an encapsulated "DEK-
   Info:" header field.

   Only one alternative is currently defined in this category.

1.1 DES in CBC Mode (DES-CBC)

   Message text and, if required, message signatures are encrypted using
   the Data Encryption Standard (DES) algorithm in the Cipher Block
   Chaining (CBC) mode of operation.  The DES algorithm is defined in
   FIPS PUB 46-1 [1], and is equivalent to the Data Encryption Algorithm
   (DEA) provided in ANSI X3.92-1981 [2].  The CBC mode of operation of

   DES is defined in FIPS PUB 81 [3], and is equivalent to those
   provided in ANSI X3.106 [4] and in ISO IS 8372 [5].  The character
   string "DES-CBC" within an encapsulated PEM header field indicates
   the use of this algorithm/mode combination.

   The input to the DES CBC encryption process shall be padded to a
   multiple of 8 octets, in the following manner.  Let n be the length
   in octets of the input.  Pad the input by appending 8-(n mod 8)
   octets to the end of the message, each having the value 8-(n mod 8),
   the number of octets being added.  In hexadecimal, the possible
   paddings are:  01, 0202, 030303, 04040404, 0505050505, 060606060606,
   07070707070707, and 0808080808080808.  All input is padded with 1 to
   8 octets to produce a multiple of 8 octets in length.  The padding
   can be removed unambiguously after decryption.

   The DES CBC encryption process requires a 64-bit cryptographic key.
   A new, pseudorandom key shall be generated for each ENCRYPTED PEM
   message.  Of the 64 bits, 56 are used directly by the DES CBC
   process, and 8 are odd parity bits, with one parity bit occupying the
   right-most bit of each octet.  When symmetric key management is
   employed, the setting and checking of odd parity bits is encouraged,
   since these bits could detect an error in the decryption of a DES key
   encrypted under a symmetric key management algorithm (e.g., DES ECB).
   When asymmetric key management is employed, the setting of odd parity
   bits is encouraged, but the checking of odd parity bits is
   discouraged, in order to facilitate interoperability, and since an
   error in the decryption of a DES key can be detected by other means
   (e.g., an incorrect PKCS #1 encryption-block format).  In all cases,
   the encrypted form of a DES key shall carry all 64 bits of the key,
   including the 8 parity bits, though those bits may have no meaning.

   The DES CBC encryption process also requires a 64-bit Initialization
   Vector (IV).  A new, pseudorandom IV shall be generated for each
   ENCRYPTED PEM message.  Section 4.3.1 of [7] provides rationale for
   this requirement, even given the fact that individual DES keys are
   generated for individual messages.  The IV is transmitted with the
   message within an encapsulated PEM header field.

   When this algorithm/mode combination is used for message text
   encryption, the "DEK-Info:" header field carries exactly two
   arguments.  The first argument identifies the DES CBC algorithm/mode
   using the character string defined above.  The second argument
   contains the IV, represented as a contiguous string of 16 ASCII
   hexadecimal digits.

   When symmetric key management is employed with this algorithm/mode
   combination, a symmetrically encrypted DES key will be represented in
   the third argument of a "Key-Info:" header field as a contiguous

   string of 16 ASCII hexadecimal digits (corresponding to a 64-bit
   key).

   To avoid any potential ambiguity regarding the ordering of the octets
   of a DES key that is input as a data value to another encryption
   process (e.g., RSAEncryption), the following holds true.  The first
   (or left-most displayed, if one thinks in terms of a key's "print"
   representation) (For purposes of discussion in this document, data
   values are normalized in terms of their "print" representation.  For a
   octet stream, the "first" octet would appear as the one on the "left",
   and the "last" octet would appear on the "right".) octet of the key
   (i.e., bits 1-8 per FIPS PUB 46-1), when considered as a data value,
   has numerical weight 2**56.  The last (or right-most displayed) octet
   (i.e., bits 57-64 per FIPS PUB 46-1) has numerical weight 2**0.

2. Message Integrity Check Algorithms

   This section identifies the alternative algorithms that shall be used
   to compute Message Integrity Check (MIC) values for PEM messages.
   Character string identifiers and ASN.1 object identifiers are
   assigned for incorporation in encapsulated "MIC-Info:" and "Key-
   Info:" header fields to indicate the choice of MIC algorithm
   employed.

   A compliant PEM implementation shall be able to process all of the
   alternative MIC algorithms defined here on incoming messages.  It is
   a sender option as to which alternative is employed on an outbound
   message.

2.1 RSA-MD2 Message Digest Algorithm

   The RSA-MD2 message digest is computed using the algorithm defined in
   RFC 1319 [9].  ( An error has been identified in RFC 1319.  The
   statement in the text of Section 3.2 which reads "Set C[j] to S[c xor
   L]" should read "Set C[j] to S[c xor L] xor C[j]".  Note that the C
   source code in the appendix of RFC 1319 is correct.)  The character
   string "RSA-MD2" within an encapsulated PEM header field indicates the
   use of this algorithm.  Also, as defined in RFC 1319, the ASN.1 object
   identifier

     md2 OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) US(840) rsadsi(113549)
         digestAlgorithm(2) 2
     }

   identifies this algorithm.  When this object identifier is used with
   the ASN.1 type AlgorithmIdentifier, the parameters component of that
   type is the ASN.1 type NULL.

   The RSA-MD2 message digest algorithm accepts as input a message of
   any length and produces as output a 16-octet quantity.  When
   symmetric key management is employed, an RSA-MD2 MIC is encrypted by
   splitting the MIC into two 8-octet halves, independently encrypting
   each half, and concatenating the results.

   When symmetric key management is employed with this MIC algorithm,
   the symmetrically encrypted MD2 message digest is represented in a
   the fourth argument of a "Key-Info:" header field as a contiguous
   string of 32 ASCII hexadecimal digits (corresponding to a 128-bit MD2
   message digest).

   To avoid any potential ambiguity regarding the ordering of the octets
   of an MD2 message digest that is input as a data value to another
   encryption process (e.g., RSAEncryption), the following holds true.
   The first (or left-most displayed, if one thinks in terms of a
   digest's "print" representation) octet of the digest (i.e., digest[0]
   as specified in RFC 1319), when considered as an RSA data value, has
   numerical weight 2**120.  The last (or right-most displayed) octet
   (i.e., digest[15] as specified in RFC 1319) has numerical weight
   2**0.

2.2 RSA-MD5 Message Digest Algorithm

   The RSA-MD5 message digest is computed using the algorithm defined in
   RFC 1321 [10].  The character string "RSA-MD5" within an encapsulated
   PEM header field indicates the use of this algorithm.  Also, as
   defined in RFC 1321, the object identifier

     md5 OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) US(840) rsadsi(113549)
         digestAlgorithm(2) 5
     }

   identifies this algorithm.  When this object identifier is used with
   the ASN.1 type AlgorithmIdentifier, the parameters component of that
   type is the ASN.1 type NULL.

   The RSA-MD5 message digest algorithm accepts as input a message of
   any length and produces as output a 16-octet quantity.  When
   symmetric key management is employed, an RSA-MD5 MIC is encrypted by
   splitting the MIC into two 8-octet halves, independently encrypting
   each half, and concatenating the results.

   When symmetric key management is employed with this MIC algorithm,
   the symmetrically encrypted MD5 message digest is represented in the
   fourth argument of a "Key-Info:" header field as a contiguous string
   of 32 ASCII hexadecimal digits (corresponding to a 128-bit MD5

   message digest).

   To avoid any potential ambiguity regarding the ordering of the octets
   of a MD5 message digest that is input as an RSA data value to the RSA
   encryption process, the following holds true.  The first (or left-
   most displayed, if one thinks in terms of a digest's "print"
   representation) octet of the digest (i.e., the low-order octet of A
   as specified in RFC 1321), when considered as an RSA data value, has
   numerical weight 2**120.  The last (or right-most displayed) octet
   (i.e., the high-order octet of D as specified in RFC 1321) has
   numerical weight 2**0.

3. Symmetric Key Management Algorithms

   This section identifies the alternative algorithms and modes that
   shall be used when symmetric key management is employed, to encrypt
   data encryption keys (DEKs) and message integrity check (MIC) values.
   Character string identifiers are assigned for incorporation in
   encapsulated "Key-Info:" header fields to indicate the choice of
   algorithm employed.

   All alternatives presently defined in this category correspond to
   different usage modes of the DES algorithm, rather than to other
   algorithms.

   When symmetric key management is employed, the symmetrically
   encrypted DEK and MIC, carried in the third and fourth arguments of a
   "Key-Info:" header field, respectively, are each represented as a
   string of contiguous ASCII hexadecimal digits.  The manner in which
   to use the following symmetric encryption algorithms and the length
   of the symmetrically encrypted DEK and MIC may vary depending on the
   length of the underlying DEK and MIC.  Section 1, Message Encryption
   Algorithms, and Section 2, Message Integrity Check Algorithms,
   provide information on the proper manner in which a DEK and MIC,
   respectively, are symmetrically encrypted when the size of the DEK or
   MIC is not equal to the symmetric encryption algorithm's input block
   size.  These sections also provide information on the proper format
   and length of the symmetrically encrypted DEK and MIC, respectively.

3.1 DES in ECB Mode (DES-ECB)

   The DES algorithm in Electronic Codebook (ECB) mode [1][3] is used
   for DEK and MIC encryption when symmetric key management is employed.
   The character string "DES-ECB" within an encapsulated PEM header
   field indicates use of this algorithm/mode combination.

   A compliant PEM implementation supporting symmetric key management
   shall support this algorithm/mode combination.

3.2 DES in EDE Mode (DES-EDE)

   The DES algorithm in Encrypt-Decrypt-Encrypt (EDE) multiple
   encryption mode, as defined by ANSI X9.17 [6] for encryption and
   decryption with pairs of 64-bit keys, may be used for DEK and MIC
   encryption when symmetric key management is employed.  The character
   string "DES-EDE" within an encapsulated a PEM header field indicates
   use of this algorithm/mode combination.

   A compliant PEM implementation supporting symmetric key management
   may optionally support this algorithm/mode combination.

4. Asymmetric Key Management Algorithms

   This section identifies the alternative asymmetric keys and the
   alternative asymmetric key management algorithms with which those
   keys shall be used, namely the asymmetric encryption algorithms with
   which DEKs and MICs are encrypted, and the asymmetric signature
   algorithms with which certificates and certificate revocation lists
   (CRLs) are signed.

4.1 Asymmetric Keys

   This section describes the asymmetric keys that shall be used with
   the asymmetric encryption algorithms and the signature algorithms
   described later.  ASN.1 object identifiers are identified for
   incorporation in a public-key certificate to identify the
   algorithm(s) with which the accompanying public key is to be
   employed.

4.1.1 RSA Keys

   An RSA asymmetric key pair is comprised of matching public and
   private keys.

   An RSA public key consists of an encryption exponent e and an
   arithmetic modulus n, which are both public quantities typically
   carried in a public-key certificate.  For the value of e, Annex C to
   X.509 suggests the use of Fermat's Number F4 (65537 decimal, or
   1+2**16) as a value "common to the whole environment in order to
   reduce transmission capacity and complexity of transformation", i.e.,
   the value can be transmitted as 3 octets and at most seventeen (17)
   multiplications are required to effect exponentiation.  As an
   alternative, the number three (3) can be employed as the value for e,
   requiring even less octets for transmission and yielding even faster
   exponentiation.  For purposes of PEM, the value of e shall be either
   F4 or the number three (3).  The use of the number three (3) for the
   value of e is encouraged, to permit rapid certificate validation.

   An RSA private key consists of a decryption exponent d, which should
   be kept secret, and the arithmetic modulus n.  Other values may be
   stored with a private key to facilitate efficient private key
   operations (see PKCS #1 [11]).

   For purposes of PEM, the modulus n may vary in size from 508 to 1024
   bits.

   Two ASN.1 object identifiers have been defined to identify RSA public
   keys.  In Annex H of X.509 [8], the object identifier

     rsa OBJECT IDENTIFIER ::= {
         joint-iso-ccitt(2) ds(5) algorithm(8)
         encryptionAlgorithm(1) 1
     }

   is defined to identify an RSA public key.  A single parameter,
   KeySize, the length of the public key modulus in bits, is defined for
   use in conjunction with this object identifier.  When this object
   identifier is used with the ASN.1 type AlgorithmIdentifier, the
   parameters component of that type is the number of bits in the
   modulus, ASN.1 encoded as an INTEGER.

   Alternatively, in PKCS #1 [11], the ASN.1 object identifier

     rsaEncryption OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
         pkcs-1(1) 1
     }

   is defined to identify both an RSA public key and the RSAEncryption
   process.  There are no parameters defined in conjunction with this
   object identifier, hence, when it is used with the ASN.1 type
   AlgorithmIdentifier, the parameters component of that type is the
   ASN.1 type NULL.

   A compliant PEM implementation may optionally generate an RSA
   public-key certificate that identifies the enclosed RSA public key
   (within the SubjectPublicKeyInformation component) with either the
   "rsa" or the "rsaEncryption" object identifier.  Use of the "rsa"
   object identifier is encouraged, since it is, in some sense, more
   generic in its identification of a key, without indicating how the
   key will be used.  However, to facilitate interoperability, a
   compliant PEM implementation shall accept RSA public-key certificates
   that identify the enclosed RSA public key with either the "rsa" or
   the "rsaEncryption" object identifier.  In all cases, an RSA public
   key identified in an RSA public-key certificate with either the "rsa"
   or "rsaEncryption" object identifier, shall be used according to the

procedures defined below for asymmetric encryption algorithms and
asymmetric signature algorithms.

4.2 Asymmetric Encryption Algorithms

   This section identifies the alternative algorithms that shall be used
   when asymmetric key management is employed, to encrypt DEKs and MICs.
   Character string identifiers are assigned for incorporation in "MIC-
   Info:" and "Key-Info:" header fields to indicate the choice of
   algorithm employed.

   Only one alternative is presently defined in this category.

4.2.1 RSAEncryption

   The RSAEncryption public-key encryption algorithm, defined in PKCS #1
   [11], is used for DEK and MIC encryption when asymmetric key
   management is employed.  The character string "RSA" within a "MIC-
   Info:" or "Key-Info:" header field indicates the use of this
   algorithm.

   All PEM implementations supporting asymmetric key management shall
   support this algorithm.

   As described in PKCS #1, all quantities input as data values to the
   RSAEncryption process shall be properly justified and padded to the
   length of the modulus prior to the encryption process.  In general,
   an RSAEncryption input value is formed by concatenating a leading
   NULL octet, a block type BT, a padding string PS, a NULL octet, and
   the data quantity D, that is,

     RSA input value = 0x00 || BT || PS || 0x00 || D.

   To prepare a DEK for RSAEncryption, the PKCS #1 "block type 02"
   encryption-block formatting scheme is employed.  The block type BT is
   a single octet containing the value 0x02 and the padding string PS is
   one or more octets (enough octets to make the length of the complete
   RSA input value equal to the length of the modulus) each containing a
   pseudorandomly generated, non-zero value.  For multiple recipient
   messages, a different, pseudorandom padding string should be used for
   each recipient.  The data quantity D is the DEK itself, which is
   right-justified within the RSA input such that the last (or rightmost
   displayed, if one thinks in terms of the "print" representation)
   octet of the DEK is aligned with the right-most, or least-
   significant, octet of the RSA input.  Proceeding to the left, each of
   the remaining octets of the DEK, up through the first (or left-most
   displayed) octet, are each aligned in the next more significant octet
   of the RSA input.

   To prepare a MIC for RSAEncryption, the PKCS #1 "block type 01"
   encryption-block formatting scheme is employed.  The block type BT is
   a single octet containing the value 0x01 and the padding string PS is
   one or more octets (enough octets to make the length of the complete
   RSA input value equal to the length of the modulus) each containing
   the value 0xFF.  The data quantity D is comprised of the MIC and the
   MIC algorithm identifier which are ASN.1 encoded as the following
   sequence.

     SEQUENCE {
         digestAlgorithm   AlgorithmIdentifier,
         digest            OCTET STRING
     }

   The ASN.1 type AlgorithmIdentifier is defined in X.509 as follows.

     AlgorithmIdentifier ::= SEQUENCE {
         algorithm         OBJECT IDENTIFIER,
         parameters        ANY DEFINED BY algorithm OPTIONAL
     }

   An RSA input block is encrypted using the RSA algorithm with the
   first (or left-most) octet taken as the most significant octet, and
   the last (or right-most) octet taken as the least significant octet.
   The resulting RSA output block is interpreted in a similar manner.

   When RSAEncryption is used to encrypt a DEK, the second argument in a
   "MIC-Info:" header field, an asymmetrically encrypted DEK, is
   represented using the printable encoding technique defined in Section
   4.3.2.4 of RFC 1421 [12].

   When RSAEncryption is used to sign a MIC, the third argument in a
   "MIC-Info:" header field, an asymmetrically signed MIC, is
   represented using the printable encoding technique defined in Section
   4.3.2.4 of RFC 1421.

4.3 Asymmetric Signature Algorithms

   This section identifies the alternative algorithms which shall be
   used to asymmetrically sign certificates and certificate revocation
   lists (CRLs) in accordance with the SIGNED macro defined in Annex G
   of X.509.  ASN.1 object identifiers are identified for incorporation
   in certificates and CRLs to indicate the choice of algorithm
   employed.

   Only one alternative is presently defined in this category.

4.3.1 md2WithRSAEncryption

   The md2WithRSAEncryption signature algorithm is used to sign
   certificates and CRLs.  The algorithm is defined in PKCS #1 [11].  It
   combines the RSA-MD2 message digest algorithm described here in
   Section 2.2 with the RSAEncryption asymmetric encryption algorithm
   described here in Section 4.2.1.  As defined in PKCS #1, the ASN.1
   object identifier

     md2WithRSAEncryption OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
         pkcs-1(1) 2
     }

   identifies this algorithm.  When this object identifier is used with
   the ASN.1 type AlgorithmIdentifier, the parameters component of that
   type is the ASN.1 type NULL.

   There is some ambiguity in X.509 regarding the definition of the
   SIGNED macro and, in particular, the representation of a signature in
   a certificate or a CRL.  The interpretation selected for PEM requires
   that the data to be signed (in our case, an MD2 message digest) is
   first ASN.1 encoded as an OCTET STRING and the result is encrypted
   (in our case, using RSAEncryption) to form the signed quantity, which
   is then ASN.1 encoded as a BIT STRING.

5. Descriptive Grammar

   ; Addendum to PEM BNF representation, using RFC 822 notation
   ; Provides specification for official PEM cryptographic algorithms,
   ; modes, identifiers and formats.

   ; Imports <hexchar> and <encbin> from RFC [1421]

       <dekalgid> ::= "DES-CBC"
       <ikalgid>  ::= "DES-EDE" / "DES-ECB" / "RSA"
       <sigalgid> ::= "RSA"
       <micalgid> ::= "RSA-MD2" / "RSA-MD5"

       <dekparameters> ::= <DESCBCparameters>
       <DESCBCparameters> ::= <IV>
       <IV> ::= <hexchar16>

       <symencdek> ::= <DESECBencDESCBC> / <DESEDEencDESCBC>
       <DESECBencDESCBC> ::= <hexchar16>
       <DESEDEencDESCBC> ::= <hexchar16>

       <symencmic> ::= <DESECBencRSAMD2> / <DESECBencRSAMD5>

       <DESECBencRSAMD2> ::= 2*2<hexchar16>
       <DESECBencRSAMD5> ::= 2*2<hexchar16>

       <asymsignmic> ::= <RSAsignmic>
       <RSAsignmic> ::= <encbin>

       <asymencdek> ::= <RSAencdek>
       <RSAencdek> ::= <encbin>

       <hexchar16> ::= 16*16<hexchar>

References

   [1] Federal Information Processing Standards Publication (FIPS PUB)
       46-1, Data Encryption Standard, Reaffirmed 1988 January 22
       (supersedes FIPS PUB 46, 1977 January 15).

   [2] ANSI X3.92-1981, American National Standard Data Encryption
       Algorithm, American National Standards Institute, Approved 30
       December 1980.

   [3] Federal Information Processing Standards Publication (FIPS PUB)
       81, DES Modes of Operation, 1980 December 2.

   [4] ANSI X3.106-1983, American National Standard for Information
       Systems - Data Encryption Algorithm - Modes of Operation,
       American National Standards Institute, Approved 16 May 1983.

   [5] ISO 8372, Information Processing Systems: Data Encipherment:
       Modes of Operation of a 64-bit Block Cipher.

   [6] ANSI X9.17-1985, American National Standard, Financial
       Institution Key Management (Wholesale), American Bankers
       Association, April 4, 1985, Section 7.2.

   [7] Voydock, V. L. and Kent, S. T., "Security Mechanisms in High-
       Level Network Protocols", ACM Computing Surveys, Vol. 15, No. 2,
       June 1983, pp. 135-171.

   [8] CCITT Recommendation X.509, "The Directory - Authentication
       Framework", November 1988, (Developed in collaboration, and
       technically aligned, with ISO 9594-8).

   [9] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319, RSA
       Laboratories, April 1992.

  [10] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
       Laboratory for Computer Science and RSA Data Security, Inc.,

       April 1992.

  [11] PKCS #1: RSA Encryption Standard, Version 1.4, RSA Data Security,
       Inc., June 3, 1991.

  [12] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part
       I: Message Encryption and Authentication Procedures", RFC 1421,
       DEC, February 1993.

  [13] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part
       II: Certificate-Based Key Management", RFC 1422, BBN, February
       1993.

  [14] Kaliski, B., "Privacy Enhancement for Internet Electronic Mail:
       Part IV: Key Certification and Related Services", RFC 1424, RSA
       Laboratories, February 1993.

Patent Statement

   This version of Privacy Enhanced Mail (PEM) relies on the use of
   patented public key encryption technology for authentication and
   encryption.  The Internet Standards Process as defined in RFC 1310
   requires a written statement from the Patent holder that a license
   will be made available to applicants under reasonable terms and
   conditions prior to approving a specification as a Proposed, Draft or
   Internet Standard.

   The Massachusetts Institute of Technology and the Board of Trustees
   of the Leland Stanford Junior University have granted Public Key
   Partners (PKP) exclusive sub-licensing rights to the following
   patents issued in the United States, and all of their corresponding
   foreign patents:

      Cryptographic Apparatus and Method
      ("Diffie-Hellman")............................... No. 4,200,770

      Public Key Cryptographic Apparatus
      and Method ("Hellman-Merkle").................... No. 4,218,582

      Cryptographic Communications System and
      Method ("RSA")................................... No. 4,405,829

      Exponential Cryptographic Apparatus
      and Method ("Hellman-Pohlig").................... No. 4,424,414

   These patents are stated by PKP to cover all known methods of
   practicing the art of Public Key encryption, including the variations
   collectively known as El Gamal.

   Public Key Partners has provided written assurance to the Internet
   Society that parties will be able to obtain, under reasonable,
   nondiscriminatory terms, the right to use the technology covered by
   these patents.  This assurance is documented in RFC 1170 titled
   "Public Key Standards and Licenses".  A copy of the written assurance
   dated April 20, 1990, may be obtained from the Internet Assigned
   Number Authority (IANA).

   The Internet Society, Internet Architecture Board, Internet
   Engineering Steering Group and the Corporation for National Research
   Initiatives take no position on the validity or scope of the patents
   and patent applications, nor on the appropriateness of the terms of
   the assurance.  The Internet Society and other groups mentioned above
   have not made any determination as to any other intellectual property
   rights which may apply to the practice of this standard. Any further
   consideration of these matters is the user's own responsibility.

Security Considerations

   This entire document is about security.

Author's Address

   David Balenson
   Trusted Information Systems
   3060 Washington Road
   Glenwood, Maryland 21738

   Phone: 301-854-6889
   EMail: balenson@tis.com